Electrode material for lithium secondary battery, electrode structure comprising the electrode material and secondary battery comprising the electrode structure

ABSTRACT

There are provided an electrode material for a lithium secondary battery which comprises alloy particles comprising silicon as a major component and having an average particle diameter of 0.02 μm to 5 μm, wherein the size of a crystallite of the alloy is not less than 2 nm but no more than 500 nm and an intermetallic compound containing at least tin is dispersed in a silicon phase and an electrode material for a lithium secondary battery which comprises alloy particles comprising silicon as a major component and having an average particle diameter of 0.02 μm to 5 μm, wherein the size of a crystallite of the alloy is not less than 2 nm but no more than 500 nm and an at least one intermetallic compound containing at least one element selected from the group consisting of aluminum, zinc, indium, antimony, bismuth and lead is dispersed in a silicon phase. Thereby, an electrode material for a lithium secondary battery, an electrode structure comprising the electrode material and a secondary battery comprising the electrode structure are provided in which a drop in capacity due to repeated charging/discharging is small, and the charge/discharge cycle life is improved.

This application is a divisional of application Ser. No. 10/809,483,filed Mar. 26, 2004, the contents of which are incorporated herein byreference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an electrode material for a lithiumsecondary battery that comprises a powder of particles comprisingsilicon as a major component, an electrode structure comprising theelectrode material and a secondary battery comprising the electrodestructure.

2. Related Background Art

Recently, it has been said that because the amount of CO₂ gas containedin the air is increasing, global warming may be occurring due to thegreenhouse effect. Thermal power plants use fossil fuels to convert athermal energy into an electric energy, however they exhaust a largeamount of CO₂ gas, thereby making it difficult to newly constructthermal power plants. Accordingly, for effective use of an electricpower generated in thermal power plants, the so-called load levelingapproach has been proposed wherein an electric power generated at night,which is an excess power, may be stored in a household secondary batteryor the like, whereby the stored electric power can be used during thedaytime when electric power consumption increases.

In addition, the development of a high energy-density secondary batteryhas been demanded for electric vehicles that do not exhaust airpollutants such as CO_(x), NO_(x), and hydrocarbons. Further, thedevelopment of compact, lightweight, high performance secondarybatteries is urgently demanded for applications in portable electricalequipment such as notebook personal computers, video cameras, digitalcameras, mobile phones, PDAs (Personal Digital Assistant) or the like.

As such a lightweight, compact secondary battery, a rocking chair typebattery referred to as “lithium ion battery” which, during a chargingreaction, uses a lithium intercalation compound as a positive electrodesubstance for allowing lithium ions to be deintercalated from betweenlayers thereof and uses a carbonaceous material represented by graphiteas a negative electrode substance for allowing lithium ions to beintercalated between planar layers of a 6-membered network-structureformed of carbon atoms have been developed and partly put into practicaluse.

However, with this “lithium ion battery”, because the negative electrodeformed of a carbonaceous material can theoretically intercalate only amaximum of ⅙ of a lithium atom per one carbon atom, a highenergy-density secondary battery comparable with a lithium primarybattery when using metallic lithium as a negative electrode material hasnot been realized.

If an amount of lithium more than the theoretical amount is tried to beintercalated in a negative electrode comprising carbon of a “lithium ionbattery” during charging or charging is performed under a high currentdensity condition, there is a possibility that lithium metal may grow ina dendrite shape on the carbon negative electrode surface, resulting inan internal short-circuit between the negative and the positiveelectrodes due to repeated charge/discharge cycles, so that any “lithiumion battery” which has a capacity more than the theoretical capacity ofa graphite negative electrode has not provided a sufficient cycle life.

On the other hand, a high-capacity lithium secondary battery that usesmetal lithium for a negative electrode has been drawing attention butnot put in practical use yet.

This is because the charge/discharge cycle life is very short. Thisshort charge/discharge cycle life is considered to be ascribed to thefact that metal lithium reacts with impurities such as water or organicsolvents contained in the electrolyte to form an insulating film or thatthe surface of a metallic lithium foil is not flat and has a portion atwhich an electric field is concentrated, whereby repeatedcharging/discharging causes lithium to grow in a dendrite shape,resulting in an internal short-circuit between the negative and positiveelectrodes, thereby leading to the end of the battery life.

In order to suppress the progress of the reaction in which metal lithiumreacts with water or organic solvents contained in the electrolyte,which is a problem peculiar to the secondary battery using a metallithium negative electrode, a method which uses a lithium alloycontaining lithium, aluminum and the like as a negative electrode hasbeen proposed.

However, this method is not currently in wide practical use because thelithium alloy is too hard to wind in a spiral form, and therefore aspiral-wound type cylindrical battery cannot be made, because the cyclelife is not sufficiently long, and because an energy density comparableto that of a battery using metal lithium for a negative electrode cannotsufficiently be obtained.

In order to resolve the above-mentioned problems, heretofor, U.S. Pat.Nos. 6,051,340, 5,795,679, and 6,432,585, Japanese Patent ApplicationLaid-Open Nos. 11-283627 and 2000-311681 and International PublicationWO 00/17949 have proposed a secondary battery that uses a negativeelectrode for a lithium secondary battery comprised of elemental tin orsilicon.

U.S. Pat. No. 6,051,340 has proposed a lithium secondary battery thatuses a negative electrode comprising an electrode layer formed of ametal that is alloyable with lithium such as silicon or tin and a metalthat is not alloyable with lithium on a current collector of a metalmaterial that is not alloyable with lithium.

U.S. Pat. No. 5,795,679 proposes a lithium secondary battery using anegative electrode formed of a powder of an alloy of an element such asnickel or copper with an element such as tin. U.S. Pat. No. 6,432,585proposes a lithium secondary battery that uses a negative electrode withan electrode material layer containing 35% or more by weight ofparticles comprised of silicon or tin with a average particle diameterof 0.5 to 60 μm and having a void ratio of 0.10 to 0.86 and a density of1.00 to 6.56 g/cm³.

Japanese Patent Application Laid-Open No. 11-283627 proposes a lithiumsecondary battery that uses a negative electrode comprising silicon ortin having an amorphous phase; Japanese Patent Application Laid-Open No.2000-311681 proposes a lithium secondary battery that uses a negativeelectrode comprising amorphous tin-transition metal alloy particles witha non-stoichiometric composition; and International Publication WO00/17949 proposes a lithium secondary battery using a negative electrodecomprising amorphous silicon-transition metal alloy particles with anon-stoichiometric composition.

However, in the lithium secondary batteries according to theabove-mentioned proposals, the efficiency of the electricity amountinvolved in lithium release relative to the electricity amount involvedin a first lithium insertion does not reach the same level ofperformance as a graphite negative electrode, so that furtherimprovement in the efficiency have been expected. In addition, since theresistances of the electrodes of the lithium secondary batteries of theabove proposals are higher than that of a graphite electrode, loweringin resistance has been desired.

Japanese Patent Application Laid-Open No. 2000-215887 proposes ahigh-capacity, high charging/discharging efficiency lithium secondarybattery in which a carbon layer is formed on the surface of particles ofa metal or semi-metal which is alloyable with lithium, in particularsilicon particles, through chemical vapor disposition using thermaldecomposition of benzene or the like to improve electrical conductivity,thereby suppressing volume expansion when alloying with lithium toprevent breakage of an electrode.

However, with this lithium secondary battery, while the theoreticalcharge capacity calculated for Li_(4.4)Si as a silicon/lithium compoundis 4200 mAh/g, an electrode performance allowing lithiuminsertion/release of an electricity amount exceeding 1000 mAh/g has notbeen attained, so that development of a high-capacity, long lifenegative electrode has been desired.

SUMMARY OF THE INVENTION

The present invention has been accomplished in view of theaforementioned problems, and it is an object of the present invention toprovide an electrode material for a lithium secondary battery in whichcapacity drop due to repeated charging/discharging is small, andcharge/discharge cycle life is improved, an electrode structurecomprising the electrode material, and a secondary battery comprisingthe electrode structure.

A first aspect of the present invention is an electrode material for alithium secondary battery comprising alloy particles comprising siliconas a major component and having an average particle diameter of 0.02 μmto 5 μm, wherein the size of a crystallite of the alloy is not less than2 nm but no more than 500 nm and an intermetallic compound containing atleast tin is dispersed in a silicon phase (First Invention).

A second aspect of the present invention is an electrode material for alithium secondary battery comprising alloy particles comprising siliconas a major component and having an average particle diameter of 0.02 μmto 5 μm, wherein the size of a crystallite of the alloy is not less than2 nm but no more than 500 nm and an at least one intermetallic compoundcontaining at least one element selected from the group consisting ofaluminum, zinc, indium, antimony, bismuth and lead is dispersed in asilicon phase.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic sectional view of a particle of an electrodematerial constituting the electrode material structure according to thepresent invention;

FIGS. 2A and 2B are conceptual views schematically illustrating sectionsof an electrode structure comprising the negative electrode material ofthe lithium secondary battery according to an embodiment of the presentinvention;

FIG. 3 is a conceptual view schematically illustrating a section of asecondary battery (lithium secondary battery) of an embodiment of thepresent invention;

FIG. 4 is a cross-sectional view of a single layer, flat type (cointype) battery;

FIG. 5 is a cross-sectional view of a spiral-wound type cylindricalbattery;

FIG. 6 is a scanning electron microscope photograph of the electrodematerial prepared in Example 1 of the present invention;

FIG. 7 is a view illustrating an X-ray diffraction profile of theelectrode material prepared in Example 1 of the present invention;

FIG. 8 is a view illustrating a selected-area electron diffraction imageof the electrode material prepared in Example 1 of the presentinvention;

FIG. 9 is a transmission electron microscope photograph of the electrodematerial prepared in Example 1 of the present invention;

FIG. 10 is a transmission electron microscope photograph of theelectrode material prepared in Reference Example 1;

FIG. 11 is views illustrating the results of elemental mapping by meansof the energy dispersive X-ray spectroscopy (EDXS) analysis of theelectrode material prepared in Example 1 of the present invention;

FIG. 12 is views illustrating the results of elemental mapping by meansof EDXS analysis of the electrode material prepared in Reference Example1; and

FIG. 13 is a graphical representation showing the results ofrelease/insertion cycle tests of the electrodes prepared in Examples 1to 4 of the present invention and Reference Examples 1 to 4.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Hereinafter, embodiments of the present invention will be explained withreference to the drawings.

The present inventors have previously found that by adding tin or copperto silicon and using a fine powder wherein the average particle diameterof alloy particles comprising 50% or more by weight of silicon elementis not less than 0.1 μm but no more than 2.5 μm, a high-capacity lithiumsecondary battery can be manufactured.

The present inventors have newly found that with an electrode materialin which an intermetallic compound comprising tin or at least oneintermetallic compound comprising at least one element selected from thegroup consisting of aluminum, zinc, indium, antimony, bismuth and leadis dispersed in a silicon phase having a crystallite size of not lessthan 2 nm and not more than 500 nm, the capacity drop due to repeatedcharging/discharging can further be reduced and the charge/dischargecycle life can be improved, to accomplish the present invention.

FIG. 1 is a schematic sectional view of a particle of an electrodematerial that constitutes an electrode structure according to thepresent invention, in which reference numeral 103 denotes a particle ofthe electrode material (active material) comprising silicon as a majorcomponent according to the present invention. The average particlediameter of this electrode material particle 103 is 0.02 μm to 5 μm.Further, this electrode material particle 103 is comprised of a siliconphase 106 and an intermetallic compound 107 which contains tin or anelement selected from the group consisting of aluminum, zinc, indium,antimony, bismuth and lead.

That is, the electrode material 103 of the present invention ischaracterized in that an intermetallic compound 107 comprising tin or anintermetallic compound 107 comprising an element selected from the groupconsisting of aluminum, zinc, indium, antimony, bismuth and lead isdisperses in a silicon phase having a crystallite size of 2 nm or moreand 500 nm or less. Here, in addition to the intermetallic compound 107,tin or the element selected from the group consisting of aluminum, zinc,indium, antimony, bismuth and lead may also be present in an elementalmetal state.

The state “intermetallic compound 107 is dispersed in silicon phase 106”referred to herein is not intended to mean that the powder particle isformed in a state of segregation in which the silicon phase 106 and aphase of the intermetallic compound 107 are separated from each otherbut is intended to mean the state such that the major component of thepowder particle is silicon and the intermetallic compound 107 is presentas a mixture therein. Further, such a state can be observed by means oftransmission electron microscope or selected-area electron diffraction.

Elements that can form an intermetallic compound with tin are preferablycopper, nickel, cobalt, iron, manganese, vanadium, molybdenum, niobium,tantalum, titanium, zircon, yttrium, lanthanum, selenium, magnesium andsilver. Of those, copper, nickel and cobalt are more preferable. Withtin these form intermetallic compounds such as Cu₄₁Sn₁₁, Cu₁₀Sn₃,Cu₅Sn₄, Cu₅Sn, Cu₃Sn, Ni₃Sn₄, Ni₃Sn₂, Ni₃Sn, CO₃Sn₂, CoSn₂, CoSn,Fe₅Sn₃, Fe₃Sn₂, FeSn₂, FeSn, Mn₃Sn, Mn₂Sn, MnSn₂, Sn₃V₂, SnV₃, Mo₃Sn,Mo₂Sn₃, MoSn₂, NbSn₂, Nb₆Sn₅, Nb₃Sn, SnTa₃, Sn₃Ta₂, SnTi₂, SnTi₃,Sn₃Ti₅, Sn₅Ti₆, SnZr₄, Sn₂Zr, Sn₃Zr₅, Sn₂Y, Sn₃Y, Sn₃Y₅, Sn₄Y₅, Sn₁₀Y₁₁,LaSn, LaSn₃, La₂Sn₃, La₃Sn, La₃Sn₅, La₅Sn₃, La₅Sn₄, La₁₁Sn₁₀, Ce₃Sn,Ce₅Sn₃, Ce₅Sn₄, Ce₁₁Sn₁₀, Ce₃Sn₅, Ce₃Sn₇, Ce₂Sn₅, CeSn₃, Mg₂Sn, Ag₃Sn,and Ag₇Sn.

Meanwhile, when silicon is used as an electrode material, because thevolume change at the time of the reaction of insertion into silicon andrelease from silicon of lithium involved in charging/discharging islarge, the crystalline structure of silicon will rupture to convert theparticles into a fine powder, so that the charging/discharging becomesunable to be performed.

Therefore, the present inventors have previously found that by usingamorphized silicon, or a fine powder of a silicon alloy, the cycle lifecan be improved, and further found that by dispersing in a silicon phasean intermetallic compound comprising tin, or an intermetallic compoundcomprising an element selected from the group consisting of aluminum,zinc, indium, antimony, bismuth and lead, lithium can be uniformlyinserted into the silicon phase, thereby improving the cycle life.

Explaining this by taking a case of tin as one example, the electricpotential E1 (Li/Li⁺) of the electrochemical oxidation/reductionreaction (1) of lithium to tin is nobler than the electric potential E2(Li/Li⁺) of the oxidation/reduction reaction (2) of lithium to silicon.Sn+xLi→Li_(x)Sn E1(Li/Li⁺)  (1)Si+xLi→Li_(x)Si E2(Li/Li⁺)  (2)E1(Li/Li⁺)>E2(Li/Li⁺)

Here, since the lithium insertion reaction involved in charging beginsfrom the nobler potential side, it is considered that the lithiuminsertion begins with tin followed by silicon. Therefore, it isconsidered that by uniformly dispersing tin in the silicon phase, thelithium insertion reaction into the silicon phase occurs uniformly, sothat uniform incorporation of lithium into the silicon phase makes itpossible to suppress the breakage of the crystalline structure ofsilicon.

Meanwhile, industrially convenient means for preparing an alloyed powderof silicon and tin include the so-called gas atomization method thatperforms alloying by atomizing a mixed and molten material, or the wateratomization method. However, because there is a large difference inmelting point such that while silicon has a melting point of 1412° C.,tin has a melting point of 231.9° C., the alloy powder is liable to beformed in a state such that a silicon phase and a tin phase are separatefrom each other.

As means to suppress this, as shown in First Invention, it is effectiveto use an intermetallic compound comprising tin.

Specifically, when preparing the alloy, it is effective to adopt amethod in which at least one element that forms an intermetalliccompound with tin selected from the group consisting of copper, nickel,cobalt, iron, manganese, vanadium, molybdenum, niobium, tantalum,titanium, zirconium, yttrium, lanthanum, selenium, magnesium and silveris added, along with tin.

Here, because these intermetallic compounds have a higher melting pointthan that of tin, the difference in melting point from silicon can bemade smaller, so that the tin phase and the alloy phase can uniformly bedispersed. Forming the intermetallic compound is also effective insuppressing the volume change when incorporating lithium.

Further, the elements of aluminum, zinc, indium, antimony, bismuth andlead can also electrochemically insert and release Li, and theiroxidation/reduction reaction potentials for Li are nobler than that ofsilicon as is the case with tin. Further, the melting points of theseelements, i.e., aluminum (660° C.), zinc (419.5° C.), indium (156.4°C.), antimony (630.5° C.), bismuth (271° C.) and lead (327.4° C.), arelower than that of silicon. Thus, as shown in Second Invention, byforming an intermetallic compound containing at least one of theseelements to reduce the difference in melting point from silicon, uniformdispersion can be achieved.

These intermetallic compounds include AlCu, AlCu₂, AlCu₃, Al₂Cu, Al₂Cu₃,Al₂Cu₇, Al₃Cu₇, Al₄Cu₅, CuZn, CuZn₃, CuZn₄, Cu₅Zn₈, Cu₂In, Cu₄In,Cu₇In₃, Cu₁₁In₉, Cu₂Sb, Cu₃Sb, Cu₄Sb, Cu₅Sb, Cu₁₀Sb₃, BiNi, Bi₃Ni,Bi₃Pb₇ and Pb₃Zr₅.

The content of silicon in the alloy is preferably 50% or more by weightin order to exhibit the performance of a high chargeable amount as alithium secondary battery negative electrode material. Further, theaverage particle diameter of the silicon alloy primary particles of thepresent invention is, as a lithium secondary battery negative electrodematerial, preferably within the range of 0.02 to 5.0 μm, and morepreferably within the range of 0.05 to 1.0 μm so that theelectrochemical lithium insertion/release reaction occurs rapidly anduniformly. The term “average particle diameter” used herein is intendedto mean the average primary particle diameter (average particle diameterin an non-agglomerated state).

Here, if the above average particle diameter is too small, handlingbecomes less easy, the area of contact between particles when forming anelectrode increases, thereby increasing the contact resistance. However,in the case of adopting the average particle diameter of the primaryparticles as mentioned above, making the particles larger by aggregatingthe primary particles leads to easier handling and lowering in theresistance.

In order to obtain a battery with a long life cycle, it is preferablethat the crystalline structure of a ground fine powder contains anamorphous phase. Further, when a fine powder of a negative electrodematerial prepared by the method of producing a lithium secondary batterynegative electrode material according to the present invention containsan amorphous phase, the volume expansion when alloying with lithium canbe reduced.

Further, when the ratio of the amorphous phase becomes larger, the fullwidth at half maximum of a peak of an X-ray diffraction chart, which issharp for a crystalline material, widens, becoming broader.Incidentally, the full width at half maximum of a main peak of an X-raydiffraction chart of diffraction intensity for 2θ is preferably 0.1° ormore, and more preferably 0.2° or more.

The size of the crystallite of the negative electrode material powder(powder of particles comprising silicon as a major component) preparedaccording to the present invention, in particular in a state in whichthe electrode structure has not been subjected to charging/dischargingyet (i.e., in an unused state) is preferably controlled to be not lessthan 2 nm but no more than 500 nm, more preferably controlled to be notless than 2 nm but no more than 50 nm, and most preferably controlled tobe not less than 2 nm but no more than 30 nm. By using such a finecrystalline powder, the electrochemical reaction duringcharging/discharging can be performed more smoothly, whereby thecharging capacity can be improved. Further, the distortion caused by theinsertion/release of lithium during charging/discharging can beminimized to increase the cycle life.

In the present invention, the crystallite size of the particles isdetermined using the following Scherrer equation on the basis of thefull width at half maximum of a peak and the diffraction angle of anX-ray diffraction curve using CuKα as an radiation source.Lc=0.94λ/(β cos θ)  (Scherrer Equation)

Lc: crystallite size

λ: wavelength of X-ray beam

β: full width at half maximum of peak (radian)

θ: Bragg angle of diffracted rays

Meanwhile, methods for preparing the electrode material according to thepresent invention includes the following:

(A) A method wherein silicon, tin or aluminum, zinc, indium, antimony,bismuth, lead, a transition metal, or the like are mixed and molten andthen subjected to atomization to form an alloy (e.g., gas atomization orwater atomization method);

(B) A method wherein a silicon alloy ingot prepared by mixing andmelting silicon, tin or aluminum, zinc, indium, antimony, bismuth, lead,a transition metal or the like is ground;

(C) A method wherein silicon powder, tin powder or a powder of aluminum,zinc, indium, antimony, bismuth, lead, a transition metal, or the likeare ground and mixed in an inert gas atmosphere to form an alloy(mechanical alloying); and

(D) A method wherein an alloy is formed from a gas phase by means ofplasma, electron beam, laser or induction heating using a volatilechloride (or other halides), oxide or the like.

In addition, by mechanically grinding these alloyed powders, it becomespossible to uniformly disperse in a silicon phase an intermetalliccompound comprising tin, or at least one intermetallic compoundcomprising at least one element selected from the group consisting ofaluminum, zinc, indium, antimony, bismuth and lead.

Here, as the mechanical grinding apparatus, there are preferably used aball mill such as a planetary ball mill, a vibrating ball mill, aconical mill and a tube mill; a media mill such as an attrition mill, asand grinder, an annular mill and a tower mill. The material of theballs as the above grinding media is preferably zirconia, stainlesssteel or steel.

Incidentally, the grinding may be performed in either of a wet processor dry process. In wet grinding, the alloy powder is ground in a solventor ground after a certain amount of solvent is added. The solvent usedin wet grinding may be water or an organic solvent such as alcohol,hexane, etc. Examples of alcohol include methyl alcohol, ethyl alcohol,1-propyl alcohol, 2-propyl alcohol, isopropyl alcohol, 1-butyl alcohol,2-butyl alcohol and the like.

FIGS. 2A and 2B illustrate schematically sections of an electrodestructure according to the present invention. In FIG. 2A, referencenumeral 102 denotes an electrode structure. This electrode structure 102is constituted of an electrode material layer 101 and a currentcollector 100. This electrode material layer 101 is constituted of, asillustrated in FIG. 2B, particles (active material) 103 comprisingsilicon as a major component, a conductive auxiliary material 104 and abinder 105. Incidentally, it should be noted that although in FIGS. 2Aand 2B the electrode material layer 101 is provided only on one surfaceof the current collector 100, an electrode material layer may be formedon both sides of the current collector 100 respectively, depending onthe battery configuration.

Here, the content of the conductive auxiliary material 104 is preferablynot less than 5% by weight but no more than 40% by weight, and morepreferably not less than 10% by weight but no more than 30% by weight.The content of the binder 105 is preferably not less than 2% by weightbut no more than 20% by weight, and more preferably not less than 5% byweight but no more than 15% by weight. The content of the particles(powder) 103 comprising silicon as a major component in the electrodematerial 101 is preferably within the range of 40% by weight to 93% byweight.

The conductive auxiliary material 104 used includes carbonaceousmaterials such as amorphous carbons such as acetylene black andketjenblack and graphite structure carbon, nickel, copper, silver,titanium, platinum, aluminium, cobalt, iron, chrome and the like, andespecially graphite is preferable. The shape of the conductive auxiliarymaterial may preferably be a shape selected from a spherical shape, aflake shape, a filament shape, a fiber shape, a spike shape, a needleshape, and the like. In addition, by employing two or more differentshapes of powders, the packing density when forming the electrodematerial layer can be increased, thereby reducing the impedance of theelectrode structure 102.

The material for the binder 105 may include a water-soluble polymer suchas polyvinyl alcohol, water-soluble ethylene-vinyl alcohol copolymer,polyvinyl butyral, polyethylene glycol, sodium carboxymethyl celluloseand hydroxyethyl cellulose; a fluororesin such as polyvinylidenefluoride and vinylidene fluoride-hexafluoropropylene copolymer; apolyolefin such as polyethylene and polypropylene; styrene-butadienerubber, polyamide-imide, polyimide, and polyamic acid (polyamideprecursor). Of these, when a combination of polyvinyl alcohol and sodiumcarboxymethyl cellulose, polyamide-imide or polyamic acid (polyamideprecursor) is used, the strength of the electrode increases, whereby anelectrode with an excellent charge/discharge cycle characteristic can bemanufactured.

In addition, because the current collector 100 has the role ofefficiently supplying an electric current to be consumed by theelectrode reaction during charging, or collecting an electric currentgenerated during discharging, in particular when applying the electrodestructure 102 to an negative electrode of a secondary battery, it isdesirable that the current collector 100 is formed of a material thathas a high electric conductivity and is inert to the battery reactions.Preferable materials include at least one metallic material selectedfrom the group consisting of copper, nickel, iron, stainless steel,titanium and platinum. A more preferable material is copper that isinexpensive and has a low electrical resistance.

Further, while the shape of the current collector 100 is a plate shape,this “plate shape” is, within the scope of practical use, notparticularly limited in thickness, and encompasses the so-called “foil”shape having a thickness of about 100 μm or less. As the plate shapemember, for example, a meshy, spongy or fibrous member, punching metal,or expanded metal can also be employed.

Now, a procedure for manufacturing the electrode structure 102 will beexplained.

First, the conductive auxiliary material 104 and the binder 105 aremixed with a silicon alloy powder of the present invention, to which anappropriate amount of a solvent for the binder 105 is added, followed bykneading to prepare a paste. Then, the prepared paste is applied to thecurrent collector 100 and dried to form the electrode material layer101, and pressing is then effected to adjust the thickness and densityof the electrode material layer 101 thus forming the electrode structure102.

As the above-mentioned application method, a coater coating method or ascreen printing method can be used. In addition, the above majorcomponent along with the conductive auxiliary material 104 and thebinder 105, without addition of a solvent, or the above negativeelectrode material along with the conductive auxiliary material 104alone, without addition of the binder 105, may be subject to pressureforming on the current collector to form the electrode material layer101.

Here, if the density of the electrode material layer 101 is too large,the expansion at the time of lithium insertion becomes greater, so thatpeeling off of the electrode material layer 101 from the currentcollector 100 occurs, and if the density of the electrode material layer101 is too small, the resistance of the electrode becomes greater, sothat the lowering in charging/discharging efficiency and the drop involtage of the battery at the time of discharging become greater. Forthese reasons, the density of the electrode material layer 101 accordingto the present invention is preferably within the range of 0.8 to 2.0g/cm³, and more preferably within the range of 0.9 to 1.5 g/cm³.

Incidentally, an electrode structure 102 formed only of the siliconalloy particles of the present invention without using the conductiveauxiliary material 104 and the binder 105 can be made by directlyforming an electrode material layer 101 on the current collector 100using a method such as sputtering, electron beam evaporation, clusterion beam deposition, or the like.

However, in this case, if the electrode material layer 101 is thick,peeling off is liable to occur at the interface with the currentcollector 100, so that the above-mentioned direct formation is notsuitable for formation of a thick electrode structure 102. Incidentally,in order to prevent the above peeling off, it is preferred that a metallayer or an oxide layer or a nitride layer is provided in a thickness ofa nanometer order on the current collector 100 to form an unevenness inthe surface of the current collector 100, thereby improving the adhesionat the interface. Examples of the oxide layer and nitride layerpreferably include an oxide layer or nitride layer of silicon or ametal.

Meanwhile, the secondary battery according to the present inventioncomprises a negative electrode using the electrode structure ascharacterized above, an electrolyte and a positive electrode andutilizes an oxidation reaction of lithium and a reduction reaction oflithium ions.

FIG. 3 is a view schematically showing a basic structure of the lithiumsecondary battery according to the present invention, in which referencenumeral 201 denotes a negative electrode using an electrode structure ofthe present invention, reference numeral 202 an ionic conductor,reference numeral 203 a positive electrode, reference numeral 204 anegative electrode terminal, reference numeral 205 a positive electrodeterminal and reference numeral 206 a battery case (housing).

Here, the above secondary battery is assembled in such a way that theionic conductor 202 is sandwiched and stacked between the negativeelectrode 201 and the positive electrode 203 to form an electrode group,then after this electrode group has been inserted into the battery casein dry air or a dry inert gas atmosphere in which the dew point issufficiently controlled, the electrodes 201, 203 are contacted to theelectrode terminals 204, 205, respectively and the battery case issealed.

Incidentally, when using a member having an electrolyte held in amicro-porous plastic film as the ionic conductor 202, the battery isassembled by inserting a micro-porous plastic film between the negativeelectrode 201 and the positive electrode 203 as a separator to preventshort-circuiting to form an electrode group, then inserting theelectrode group into the battery case, connecting the electrodes 201,203 to the electrode terminals 204, 205, respectively, injecting theelectrolyte and sealing the battery case.

The lithium secondary battery that uses an electrode structurecomprising an electrode material of the present invention as thenegative electrode has a high charging/discharging efficiency andcapacity and a high energy density owing to the above-mentionedadvantageous effects of the negative electrode.

Herein, the positive electrode 203, which is the counter electrode ofthe lithium secondary battery using the electrode structure of thepresent invention as the negative electrode, comprises a positiveelectrode material that is at least a lithium ion source and serves as ahost material for lithium ions, and preferably comprises a layer formedof a positive electrode material that serves as a host material forlithium ions and a current collector. Further, it is preferable that thelayer formed of the positive electrode material comprises the positiveelectrode material that serves as a host material for lithium ions and abinder, and a conductive auxiliary material as occasion demands.

As the positive electrode material that is a lithium ion source andserves as a host material used in the lithium secondary battery of thepresent invention, there are preferably included lithium-transitionmetal oxides, lithium-transition metal sulfides, lithium-transitionmetal nitrides and lithium-transition metal phosphates. The transitionmetal for the transition metal oxides, transition metal sulfides,transition metal nitrides or transition metal phosphates includes, forexample, metal elements having a d-shell or f-shell, i.e., Sc, Y,lanthanoids, actinoids, Ti, Zr, Hf, V, Nb, Ta, Cr, Mo, W, Mn, Tc, Re,Fe, Ru, Os, Co, Rh, Ir, Ni, Pb, Pt, Cu, Ag, and Au, and in particularCo, Ni, Mn, Fe, Cr, and Ti are preferably used.

Where the above positive electrode active material is a powder, thepositive electrode is made by using a binder, or made by forming thepositive electrode active material layer on the current collector bycalcination or deposition. Further, when the conductivity of the powderof the positive electrode active material is low, it becomes necessaryto suitably mix a conductive auxiliary material therewith as in theabove-mentioned formation of the active material layer for the electrodestructure. The conductive auxiliary materials and binders that may beused are the same as those mentioned above for the electrode structure102 of the present invention.

The current collector material used for the positive electrode ispreferably a material that has a high electrical conductivity and isinert to the battery reaction, such as aluminium, titanium, nickel andplatinum. Specifically, nickel, stainless steel, titanium and aluminiumare preferable, of which aluminium is more preferable because it isinexpensive and has a high electrical conductivity. Further, while theshape of the current collector is a plate shape, this “plate shape” is,within the scope of practical use, not particularly limited inthickness, and encompasses the so-called “foil” shape having a thicknessof about 100 μm or less. As the plate shape member, for example, ameshy, spongy or fibrous member, punching metal, or expanded metal canalso be employed

In addition, as the ionic conductor 202 of the lithium secondary batteryof the present invention, lithium ion conductors such as a separatorholding an electrolyte solution (electrolyte solution prepared bydissolving an electrolyte in a solvent), a solid electrolyte, or asolidified electrolyte obtained by gelling an electrolyte solution witha polymer gel, a complex of a polymer gel and a solid electrolyte can beused. Here, the conductivity of the ionic conductor 202 at 25° C. ispreferably 1×10⁻³ S/cm or more, and more preferably 5×10⁻³ S/cm or more.

As the electrolyte, there may be included salts comprised of lithiumions (Li⁺) and Lewis acid ions (BF₄ ⁻, PF₆ ⁻, AsF₆ ⁻, ClO₄ ⁻, CF₃SO₃ ⁻,or BPh₄ ⁻ (Ph: phenyl group)) and mixtures thereof. It is preferablethat the above salts have been previously subjected to sufficientdehydration and deoxidation by heating under a reduced pressure or thelike.

As a solvent for the electrolyte, there may be included, for example,acetonitrile, benzonitrile, propylene carbonate, ethylene carbonate,dimethyl carbonate, diethyl carbonate, ethylmethyl carbonate, dimethylformamide, tetrahydrofuran, nitrobenzene, dichloroethane,diethoxyethane, 1,2-dimethoxyethane, chlorobenzene, γ-butyrolactone,dioxolane, sulfolane, nitromethane, dimethyl sulfide, dimethylsulfoxide, methyl formate, 3-methyl-2-oxazolidinone,2-methyltetrahydrofulan, 3-propylsydnone, sulfur dioxide, phosphorylchloride, thionyl chloride, sulfuryl chloride or a liquid mixturethereof.

Incidentally, it is preferable to either dehydrate the above-mentionedsolvent, for example, with activated alumina, a molecular sieve,phosphorus pentaoxide or calcium chloride, or depending on the solvent,to distill the solvent in an inert gas atmosphere in the presence of analkaline metal for elimination of impurities and dehydration.

In order to prevent leakage of the electrolyte solution, it ispreferable to use a solid electrolyte or a solidified electrolyte. Thesolid electrolyte may include a glass material such as an oxide materialcomprising lithium, silicon, oxygen, and phosphorus or sulfur elements,a polymer complex of an organic polymer having an ether structure. Thesolidified electrolyte is preferably obtained by gelling the aboveelectrolyte solution with a gelling agent to solidify the electrolytesolution.

It is desirable to use as the gelling agent a polymer that can absorbthe solvent of the electrolyte solution to swell, or a porous materialcapable of absorbing a large amount of liquid, such as silica gel. Asthe polymer, there may be used polyethylene oxide, polyvinyl alcohol,polyacrylonitrile, polymethylmethacrylate,vinylidenefluoride-hexafluoropropylene copolymer, and the like. Further,it is more preferred that the polymers have a cross-linking structure.

The ionic conductor 202 constituting the separator which plays the roleof preventing short-circuiting between the negative electrode 201 andthe positive electrode 203 in the secondary battery may also have a roleof retaining the electrolyte solution and is required to have a largenumber of fine pores through which lithium ions can pass and to beinsoluble and stable in the electrolyte solution.

Accordingly, as the material of the ionic conductor 202 (separator),there are preferably used, for example, a material of a microporestructure made of glass, a polyolefin such as polypropylene orpolyethylene, a fluororesin, etc., or a nonwoven fabric. Alternatively,a metal oxide film having micropores or a resin film complexed with ametal oxide may also be used.

Now, the shape and structure of the secondary battery will be explained.

The specific shape of the secondary battery according to the presentinvention may be, for example, a flat shape, a cylindrical shape, arectangular parallelepiped shape, a sheet shape or the like. Thestructure of the battery may be, for example, a single layer type, amultiple layer type, a spiral-wound type or the like. Of those, aspiral-wound type cylindrical battery permits an enlarged electrodesurface area by rolling a separator that is sandwiched between anegative electrode and a positive electrode, thereby being capable ofsupplying a large current at the time of charging/discharging.Furthermore, batteries having a rectangular parallelepiped shape orsheet shape permit effective utilization of accommodation space inappliances that will be configured by accommodating a plurality ofbatteries therein.

Now, description will be made in more detail of the shape and structureof the battery with reference to FIGS. 4 and 5. FIG. 4 is a sectionalview of a single layer type flat (i.e., coin type) battery and FIG. 5 isa sectional view of a spiral-wound type cylindrical battery. Theselithium secondary batteries generally comprise the same structure asthat illustrated in FIG. 3, a negative electrode, a positive electrode,an electrolyte, an ionic conductor, a battery housing and an outputterminal.

In FIGS. 4 and 5, reference numerals 301, 403 denote negativeelectrodes, reference numerals 303, 406 positive electrodes, referencenumerals 304, 408 negative electrode caps or negative electrode cans asnegative electrode terminals, reference numerals 305, 409 positiveelectrode caps or positive electrode cans as positive electrodeterminals, reference numeral 302, 407 ionic conductors, referencenumerals 306, 410 gaskets, reference numeral 401 represents a negativeelectrode current collector, reference numeral 404 a positive electrodecurrent collector, reference numeral 411 an insulating plate, referencenumeral 412 a negative electrode lead, reference numeral 413 a positiveelectrode lead, and reference numeral 414 a safety valve.

In the flat secondary battery (coin type) shown in FIG. 4, the positiveelectrode 303 that contains a positive electrode material layer and thenegative electrode 301 that contains a negative electrode material layerare stacked with an ionic conductor 302 which is formed by a separatorthat retains at least an electrolyte solution therein, wherein the stackis accommodated from the positive electrode side into the positiveelectrode can 305 used as a positive terminal and the negative electrodeis covered with the negative electrode cap 304 used as a negativeelectrode. A gasket 306 is provided in the remaining portions of thepositive electrode can.

In the spiral-wound type cylindrical secondary battery shown in FIG. 5,the positive electrode 406 having a positive electrode (material) layer405 formed on the positive electrode current collector 404 and thenegative electrode 403 having the negative electrode (material) layer402 formed on the negative electrode current collector 401 are providedin opposition to each other via the ionic conductor 407 formed by aseparator that retains at least an electrolyte solution therein so as toform a stack of a cylindrical structure rolled up multiple times.

The cylindrical stack is accommodated in the negative electrode can 408used as the negative electrode terminal. Furthermore, the positiveelectrode cap 409 is disposed as the positive electrode terminal on aside of an opening of the negative electrode can 408 and a gasket 410 isdisposed in the remaining parts of the negative electrode can. Thecylindrical electrode stack is isolated from the positive electrode capside by the insulating plate 411.

The positive electrode 406 is connected to the positive electrode cap409 by way of the positive electrode lead 413. The negative electrode403 is connected to the negative electrode cap 408 by way of thenegative electrode lead 412. The safety valve 414 is disposed on theside of the positive electrode cap to adjust the internal pressure ofthe battery. As mentioned above, a layer comprising the above negativeelectrode material fine powder of the present invention is used as theactive material layer 402 of the negative electrode 403.

Next, an example of assembling procedures for the battery shown in FIGS.4 and 5 will be described.

(1) The ionic conductor 302, 407 as a separator is sandwiched betweenthe negative electrode 301, 403 and the formed positive electrode 303,406, and assembled into the positive electrode can 305 or the negativeelectrode can 408.

(2) After injection of the electrolyte solution, the negative electrodecap 304 or the positive electrode cap 409 is assembled with the gasket306, 410.

(3) The assembly obtained in (2) above is caulked.

The battery is completed in this way. Incidentally, it is preferablethat the above-described preparation of the materials for the lithiumbattery and assembly of the battery is carried out in dry air from whichmoisture has been removed sufficiently or in a dry inert gas.

Next, members comprising the secondary battery will be described.

As the material of the gasket 306, 410, there may be used, for example,a fluororesin, a polyolefin resin, a polyamide resin, a polysulfoneresin, or a rubber material. The sealing of the battery may be conductedby way of glass-sealing, sealing using an adhesive, welding orsoldering, besides the caulking using the insulating packing shown inFIG. 4 or 5. As the material of the insulating plate 411 shown in FIG.4, organic resin materials and ceramics may be used.

The battery housing is constituted of the positive electrode can 305 orthe negative electrode can 408, and the negative electrode cap 304 orthe positive electrode cap 409. As the material of the battery housing,stainless steel is preferably used. Further, as other materials of thebattery housing, there are frequently used an aluminum alloy, a titaniumclad stainless steel, a copper clad stainless steel or a nickel-platedsteel.

The positive electrode can 305 illustrated in FIG. 4 and the negativeelectrode can 408 illustrated in FIG. 5 function as the battery housing(case) and also as a terminal and is therefore preferably made ofstainless steel. However, where the positive electrode 305 or thenegative electrode 408 does not function as both the battery housing(case) and the terminal, in addition to stainless steel, a metal such aszinc, a plastic such as polypropylene, a composite material of a metalor glass fibers and a plastic may be used.

As the safety valve 414 provided in the lithium secondary battery inorder to ensure safety when the internal pressure in the battery isincreased, for example, rubber, a spring, a metal ball or a rupture diskmay be used.

EXAMPLES

In the following, the present invention will be described in more detailwith reference to examples.

(Preparation of Electrode Material)

First, examples for the preparation of a negative electrode materialwill be explained.

Example 1

65% by weight of Si, 30% by weight of Sn and 5% by weight of Cu weremelted and mixed to make an alloy, which was subjected to wateratomization to prepared a Si—Sn—Cu alloy powder having an averageparticle diameter of 10 μm. Next, the prepared alloy powder was groundwith a bead mill (ball mill using beads with comparatively smalldiameter as grinding media) to obtain a Si—Sn—Cu alloy fine powder. Thisgrinding was performed using zirconia beads in isopropyl alcohol.

Then, processing for 2 hours in a high-energy planetary-type ball millin an argon gas atmosphere using balls made of silicon nitride providedan electrode material of Si—Sn—Cu alloy fine powder.

Example 2

An electrode material of a Si—Zn—Cu alloy fine powder was obtainedfollowing the same procedure as Example 1 with the exception that analloy with a composition of 70% by weight of Si, 25% by weight of Zn and5% by weight of Cu was prepared by a gas atomization process usingnitrogen gas.

Example 3

An electrode material of a Si—Sn—Co alloy fine powder was obtainedfollowing the same procedure as Example 1 with the exception that analloy with a composition of 50% by weight of Si, 40% by weight of Sn and10% by weight of Co was prepared by a water atomization process.

Example 4

An electrode material of a Si—Sn—Ni alloy fine powder was obtainedfollowing the same procedure as Example 1 with the exception that analloy with a composition of 85% by weight of Si, 10% by weight of Sn and5% by weight of Ni was prepared by a water atomization process.

Reference Example 1

An electrode material of a Si—Sn—Cu alloy fine powder was obtainedfollowing the same procedure as Example 1 with the exception that theprocessing in a high-energy planetary-type ball mill was not performed.

Reference Example 2

An electrode material of a Si—Zn—Cu alloy fine powder was obtainedfollowing the same procedure as Example 2 with the exception that theprocessing in a high-energy planetary-type ball mill was not performed.

Reference Example 3

An electrode material of a Si—Sn—Co alloy fine powder was obtainedfollowing the same procedure as Example 3 with the exception that theprocessing in a high-energy planetary-type ball mill was not performed.

Reference Example 4

An electrode material of a Si—Sn—Ni alloy fine powder was obtainedfollowing the same procedure as Example 4 with the exception that theprocessing in a high-energy planetary-type ball mill was not performed.

Next, the results of analyzing the electrode materials obtained inExamples 1 to 4 and Reference Examples 1 to 4 will be explained.

The above Si alloy electrode materials were analyzed from the viewpointof factors that are considered to affect the performance of a negativeelectrode of a lithium secondary battery, such as average particlediameter, crystallite size, intermetallic compounds of Sn or Zn, anddistribution of elements in the alloy.

Here, the average particle diameter was determined by a laserdiffraction/scattering particle size distribution analyzer, and furtherobserved with a scanning electron microscope (SEM). Further, thecrystallite size was calculated from the full width at half maximum ofan X-ray diffraction peak in accordance with the Scherrer equation, anddetection of Sn or Zn intermetallic compounds was performed byinvestigation using the selected-area electron diffraction.

Further, the distribution of elements in the alloy was investigated byTEM observation in terms of nonuniformity in color density within thealloy particle. Incidentally, when the localization of elements in thealloy is small and the elements are uniformly dispersed, an image withless nonuniformity in color density within the alloy particle isobserved, and in elemental mapping by the energy dispersive X-rayspectroscopy (EDXS) combined with TEM, less localization of elementaldistribution within the particle is observed.

The electrode material made in Example 1 was measured for particle sizedistribution with a laser diffraction/scattering particle sizedistribution analyzer (model: LA-920 manufactured by Horiba Ltd.), withthe result that the median diameter was 0.28 μm. FIG. 6 is a photographof the electrode material obtained by SEM observation, from which it wasseen that the electrode material (negative electrode material) wereuniform particles of 0.5 μm or less.

In addition, X-ray diffraction measurement was carried out to obtain theprofile of FIG. 7. The crystallite size calculated from the Scherrerequation using the full width at half maximum of a peak at 28°±1 as amain peak of silicon was 11.1 nm.

Further, electron diffraction was performed at a selected-area region ofa diameter of 150 nm adopted in the TEM observation. The results arecollectively shown in FIG. 8. Incidentally, as to the ring diffractionpattern of FIG. 8, the calculated d values are collectively shown inTable 1.

TABLE 1 d value of Si d value of d value calculated from (JCPDS cardCu₆Sn₅ (JCPDS electron diffraction results number: 27- card number: ofmaterial made in Example 1 1402) 02-0713) 3.13 3.14 2.93 2.96 2.55 2.552.09 2.09 2.08 1.90 1.92 1.71 1.63 1.62

Thus, it was seen from Table 1 that the d values calculated from theresults of electron diffraction of the electrode material made inExample 1 were quite similar to the d values of the JCPDS card numberfor Cu₆Sn₅, which meant the presence of Cu₆Sn₅.

Further, Examples 2 to 4 were also investigated in the same manner asdescribed above, and the average particle diameter, crystallite size,and observed intermetallic compounds of the electrode materials made inExample 1 to 4 are collectively shown in Table 2.

TABLE 2 Average particle Crystallite Observed diameter sizeintermetallic (μm) (nm) compound Example 1 Si/Sn/Cu = 65/30/5 0.28 11.1Cu₆Sn₅ (weight ratio) Example 2 Si/Zn/Cu = 85/10/5 0.24 11.3 Cu₅Zn₈(weight ratio) Example 3 Si/Sn/Co = 50/40/10 0.49 11.7 CoSn, Co₃Sn₂,(weight ratio) Co₃Sn Example 4 Si/Sn/Ni = 85/10/5 0.25 10.5 Ni₃Sn₂(weight ratio)

Thus, it was seen from Table 2 that for the Si alloys made in Examples 1to 4, the average particle diameter was 0.24 to 0.49 μm, the crystallitesize was 10.5 to 11.7 nm, and further that Sn intermetallic compounds orZn intermetallic compounds were present.

Next, the elemental distributions in the alloys using the electrodematerials made in Example 1 and Reference Example 1 were investigated.FIGS. 9 and 10 are photographs obtained by TEM observation of theelectrode materials made in Example 1 and Reference Example 1. Further,FIGS. 11 and 12 show the results of elemental mapping using the EDXSanalysis.

From these results, it was seen that the portion of a low color densitywas an Si phase, and the portions of high color densities were an Snphase and a Cu₆Sn₅ phase. It was seen from FIG. 9 that the electrodematerial made in Example 1 was small in nonuniformity of color density,and therefore that the Sn phase and the Sn₆Cu₅ phase were disperseduniformly in the Si phase. In contrast, it was seen from FIG. 10 thatthe electrode material made in Reference Example 1 was large innonuniformity of color density within the alloy particle, and thereforethat the Si phase and the Sn phase and the Sn₆Cu₅ phase were presentnonuniformly within the particles.

Further, the same observation results were obtained for Example 2 andReference Example 2, Example 3 and Reference Example 3, and Example 4and Reference Example 4.

Next, as will be described below, electrode structures were manufacturedusing the fine powders of the silicon alloys obtained following theprocedures described above and evaluated for the lithiuminsertion/release performance thereof.

First, 66.5% by weight of each of the silicon alloy fine powdersobtained by the above procedure, 10.0% by weight of a flat graphitepowder as a conductive auxiliary material (specifically, graphite powderwith a substantially disk-shaped particles of a diameter of about 5 μmand a thickness of about 5 μm), 6.0% by weight of a graphite powder(substantially spherical particles with an average particle size of 0.5to 1.0 μm), 4.0% by weight of an acetylene black powder (substantiallyspherical particles with an average particle size of 4×10⁻² μm), 10.5%by weight of polyvinyl alcohol as a binder and 3.0% by weight of sodiumcarboxymethyl cellulose were mixed and kneaded with addition of water toprepare a paste.

Next, the thus prepared paste was applied on an electrical field copperfoil (electrochemically produced copper foil) of 15 μm in thickness bymeans of a coater and dried, and the thickness was adjusted with aroller press machine to obtain an electrode structure having an activematerial layer with a thickness of 25 μm.

The resultant electrode structure was cut into a shape/size of 2.5cm×2.5 cm square and a copper tub was welded thereto to obtain a siliconelectrode.

(Evaluation Procedure for Lithium Insertion/Release)

Next, a lithium metal foil of 100 μm in thickness was pressure bonded toa copper foil to make a lithium electrode. Next, ethylene carbonate anddiethyl carbonate were mixed at a volume ratio of 3:7 to obtain anorganic solvent, to which a LiPF₆ salt was dissolved at a concentrationof 1 M (mol/L) to prepare an electrolyte solution.

Then, the electrolyte solution was impregnated into a porouspolyethylene film of 25 μm in thickness. Next, the above siliconelectrode was arranged on one surface of the polyethylene film and theabove lithium electrode was arranged on the other surface of thepolyethylene film such that the polyethylene film was sandwiched by theelectrodes. In order to provide flatness, this stack was pinched by apair of glass sheets, and then covered with an aluminum laminated filmto make an evaluation cell.

This aluminum laminated film was a three-layered film consisting of anoutermost nylon film layer, a middle aluminum foil layer with athickness of 20 μm, and an inside polyethylene film layer. The outputterminal portions of the electrodes were sealed by fusion withoutlamination.

In order to evaluate the performance of the above electrode structure asa negative electrode, a lithium insertion/release cycle test(charge/discharge cycle test) was performed.

Namely, the evaluation cell was connected to a charging/dischargingapparatus with the lithium electrode being the anode and the siliconelectrode being the cathode. First, the evaluation cell was dischargedat a current density of 0.112 mA/cm² (70 mA per 1 g of the activematerial layer of the silicon electrode, that is, 70 mA/gram ofelectrode layer weight) to insert lithium into the silicon electrodelayer, then the evaluation cell was charged at a current density of 0.32mA/cm² (200 mA/gram of electrode layer weight) to release lithium fromthe silicon layer, and the electricity amount involved in lithiuminsertion/release per unit weight of the silicon electrode layer, or thesilicon powder or silicon alloy powder was evaluated at a voltage rangeof 0 to 1.2 V.

FIG. 13 is a view showing the results of the lithium insertion/releasecycle test of the electrode structures of Examples 1 to 4 and ReferenceExamples 1 to 4, wherein the abscissa indicates the number of cycles andthe ordinate represents the amount of lithium released.

As shown by FIG. 13, for the electrodes of Reference Examples 1 to 4 inwhich the intermetallic compound of Sn or Zn is not uniformly dispersein the Si phase, the amount of Li released decreases as the cycles arerepeated. However, for the electrodes of Examples 1 to 4 of the presentinvention where the intermetallic compound of Sn or Zn is uniformlydisperse in the Si phase, the amount of Li released does not decrease.Thus, it was seen that the silicon alloy electrodes made in the examplesof the present invention each had a longer life.

Next, a secondary battery was made as Example 5 of the presentinvention.

Example 5

In this example, an electrode structure having electrode layers formedon both sides of a current collector was made using a negative electrodematerial according to the present invention. The thus made electrodestructure was used as a negative electrode to make a lithium secondarybattery of a 18650 size (diameter 18 mmφ×height 65 mm) having thesectional structure as shown in FIG. 5.

1. Preparation of Negative Electrode 403

The negative electrode 403 was made according to the following procedureusing the electrode materials of Examples 1 to 4.

First, 66.5% by weight of each of the silicon alloy fine powdersobtained by the above procedure, 10.0% by weight of a flat graphitepowder as a conductive auxiliary material (specifically, graphite powderwith a substantially disk-shaped particles of a diameter of about 5 μmand a thickness of about 5 μm), 6.0% by weight of a graphite powder(substantially spherical particles with an average particle size of 0.5to 1.0 μm), 4.0% by weight of an acetylene black powder (substantiallyspherical particles with an average particle size of 4×10⁻² μm), and13.5% by weight of a binder were mixed, and N-methyl-2-pyrrolidone wasadded to prepare a paste.

Incidentally, as the binder, polyamide-imide was used for the electrodematerials of Examples 1 and 2, and polyamic acid (polyamide precursor)was used for the electrode materials of Examples 3 and 4.

Next, the thus prepared paste was applied on an electrical field copperfoil (electrochemically produced copper foil) of 15 μm in thickness bymeans of a coater and dried, and the thickness was adjusted with aroller press machine to prepare an electrode structure having an activematerial layer with a thickness of 25 μm.

The electrode structure having electrode layers provided on both sidesof the current collector according to the above procedure was cut into apredetermined size, and a lead of a nickel ribbon was connected to theelectrode by spot welding to obtain the negative electrode 403.

2. Preparation of Positive Electrode 406

(1) Lithium citrate and cobalt nitrate were mixed at a molar ratio of1:3, followed by addition of citric acid, and the resulting mixture wasthen dissolved in ion-exchanged water to obtain a solution. The solutionwas sprayed into an air stream of 200° C. to prepare a precursor of alithium-cobalt oxide fine powder.(2) The precursor of a lithium-cobalt oxide prepared in above (1) washeat-treated in an air stream at 850° C.(3) The lithium-cobalt oxide prepared in above (2) was mixed with 3% byweight of a graphite powder and 5% by weight of a polyvinylidenefluoride powder, to which N-methyl-2-pyrrolidone was then added to makea paste.(4) The paste obtained in above (3) was applied on both surfaces of analuminium foil of a thickness of 20 μm as the current collector 404,then dried and the thickness the positive electrode material layer oneach side was adjusted with a roller press machine to 90 μm. Further, analuminium lead was connected by an ultrasonic welding machine, and driedat 150° C. under a reduced pressure to prepare the positive electrode406.3. Preparation Procedure of Electrolyte Solution(1) Ethylene carbonate and diethyl carbonate whose moisture had beensufficiently removed were mixed at a volume ratio of 3:7 to prepare asolvent.(2) Into the solvent obtained in above (1) was dissolved lithiumtetrafluoroborate (LiBF₄) at a concentration of 1 M (mole/L) to obtainan electrolyte solution.4. Separator 407

A microporous polyethylene film of 25 μm in thickness was used as theseparator.

5. Battery Assembly

Assembly was entirely conducted in a dry atmosphere controlled inmoisture with a dew point of −50° C. or less.

The separator 407 was sandwiched between the negative electrode 403 andthe positive electrode 406, and the sandwiched member was then spirallywound so as to have a structure of separator/positiveelectrode/separator/negative electrode/separator, and inserted in thenegative electrode can 408 made of stainless steel.

Next, the negative electrode lead 412 was spot-welded to a bottomportion of the negative electrode can 408. A constriction was formed atan upper portion of the negative electrode can by means of a neckingmachine, and the positive electrode lead 413 was welded to the positiveelectrode cap 409 provided with a gasket 410 made of polypropylene bymeans of a spot welding machine.

(3) Next, after an electrolyte solution had been injected, the positiveelectrode cap was put on, and the positive electrode cap and thenegative electrode can were caulked with a caulking machine and sealedto prepare the battery.

Incidentally, the battery was a positive electrode capacity regulatedbattery in which the negative electrode capacity was larger than thepositive electrode capacity.

(6) Evaluation

Charging/discharging was performed for each of the batteries, and thedischarging capacity was measured.

As a result, the discharging capacities of the lithium secondarybatteries using the electrode structures formed of the electrodematerials of Examples 1 to 4 as the negative electrodes all exceeded2800 mAh. Further, even at the 100th cycle, discharging capacitiescorresponding to 75% or more of the initial capacities were maintained.

As described above, according to the preferable examples of the presentinvention, a high capacity secondary battery can be produced in which adrop in capacity due to repeated charging/discharging is small, and thecharge/discharge cycle life is improved.

1. An electrode material for a lithium secondary battery, the electrodematerial comprising alloy particles, the alloy particles comprising: asilicon phase as a major component; and an intermetallic compoundcontaining at least one element selected from the group consisting ofaluminum, zinc, indium, antimony, busmuth and lead, the intermetalliccompound being dispersed in the silicon phase, wherein the alloyparticles have an average particle diameter of 0.02 μm to 5 μm, andwherein the crystallite size of the silicon phase is not less than 2 nmbut no more than 500 nm, as determined by the Scherrer EquationLc=0.94λ/(βcosθ), where Lc is the crystallite size, λ is the wavelengthof the X-ray beam, β is the full width at half maximum of peak and θ isthe Bragg angle of diffracted rays.
 2. The electrode material for alithium secondary battery according to claim 1, wherein the alloyparticles further comprise at least one metal element present in anelemental metal state selected from the group consisting of tin,aluminum, zinc, indium, antimony, bismuth and lead.
 3. The electrodematerial for a lithium secondary battery according to claim 1, whereinthe content of silicon in the alloy particles is not less than 50% byweight but no more than 90% by weight.
 4. An electrode structurecomprising the electrode material set forth in claim 1, a conductiveauxiliary material, a binder and a current collector.
 5. The electrodestructure according to claim 4, wherein the conductive auxiliarymaterial is a carbonaceous material.
 6. A secondary battery, whichcomprises a negative electrode using the electrode structure set forthin claim 4, an electrolyte and a positive electrode, and which utilizesan oxidation reaction of lithium and a reduction reaction of lithiumions.